1. Field of the Invention
The present invention relates to image forming apparatus such as printers, copying machines, and facsimile apparatus. More specifically, the present invention is directed to image forming apparatus equipped with a member for abutting against image carriers.
2. Description of the Related Art
In image forming apparatus using an electrophotographic system such as printers, copying machines, and facsimile apparatus, image forming operations are carried out as follows. That is, after latent images are acquired by latent image forming unit in such a manner that light is irradiated onto image carriers such as photosensitive drums which have been uniformly charged, toners are applied to the charged latent images by developing unit so as to produce visible images. Then, for example, a plurality of toner images are transferred to intermediate transfer members in a multiple mode, and thereafter, these multiple toner images are transferred onto recording papers so as to be fixed thereon.
As a developing unit of this image forming apparatus, recently, a rotary type developing apparatus has been provided in which four sets of color developing devices (yellow, magenta, cyan, and black developing devices) are held in the vicinity of a single image carrier along a circumferential direction of a rotary member. In the case that this rotary type developing apparatus is employed, when a desirable developing device is moved to a developing position opposite to the image carrier, an interval between the image carrier and a developing agent carrier such as a developing roller employed in this desirable developing device must be kept constant.
To this end, in a conventional image forming apparatus, while tracking rollers are provided on both coaxial edge portions of a developing roller in each of developing devices and the tracking rollers correspond to such a positioning member capable of holding an interval between a photosensitive drum (image carrier) and an developing roller (developing agent carrier), these tracking rollers are caused to abut (is contacted to) the photosensitive drum so as to perform positioning of the developing device, so that the interval between the photosensitive drum and the developing roller can be made constant as described in JP-A-2001-183902.
In this technique, for example, since a photosensitive drum is pivotally rotated around a shaft as a center, respective image forming processes constructed of a charging process, an exposing process, a developing process, and a transferring (primary transferring) process are carried out. If the respective apparatuses for executing these image forming processes are made in no contact with respect to this photosensitive drum, then the pivotal rotation of this photosensitive drum may be controlled based upon only variable elements of the photosensitive drum. However, even in such a case that, for instance, a developing device for forming a single color image is employed in addition to such a tracking roller as described in the above-explained patent publication, when an abutting function is provided in which a predetermined member abuts against a photosensitive drum due to a predetermined positioning aspect, the following problem may occur. That is, weight produced when these members abut against the photosensitive drum may cause such a problem with respect to this photosensitive drum. This contact (abutting) problem with respect to an image carrier such as a photosensitive drum may similarly occur even in the case that a transfer apparatus such as a transfer belt which abuts against this image carrier is employed.
When these abutting members are strongly depressed with respect to the image carrier such as the photosensitive drum, there are some possibilities that the shaft of the photosensitive drum is bent due to the weight of these abutting members, and thus, this shaft bending phenomenon may cause disturbances of images, for instance, color shifts, or color deviation. This bending phenomenon may give an adverse influence to holding characteristics of the photosensitive drum, and further, this weight may give an adverse influence to such a phenomenon that the shaft of the photosensitive drum is bent in accordance with a time elapse. This problem may be conspicuously emphasized in such a case that a diameter of a drum shaft is made narrower due to a cost-down reason and/or light weight of an image forming apparatus. More specifically, in the case that such a drum holding structure that one shaft is penetrated is not employed, but a photosensitive drum is supported by two shafts (namely, cantilever shafts) which are projected from both ends of this photosensitive drum in a half way, since these cantilevers own deteriorated stabilities, abutting weight given to the photosensitive drum may readily give an adverse influence to a holding characteristic of this photosensitive drum by this shaft. Also, it is easily predictable that variations may be considerably increased due to shifts of a center shaft of this photosensitive drum.
Also, for instance, as in a rotary type developing apparatus, in the case that a member abuts against an image carrier (e.g., photosensitive drum and photosensitive belt) at predetermined timing, when this member is advanced (abuts) and is retracted (separated) to/from this image carrier, shocks and vibrations may occur. These shocks and vibrations are transferred to, for example, a primary transferring unit, a secondary transferring unit, and an exposing unit, so that disturbances of images may readily occur. An image forming apparatus may be constituted in such a manner that while timing of an image forming operation is adjusted, for instance, a tracking roller of a developing apparatus abuts against an image carrier at such a timing that no exposing operation is carried out. However, for example, in the case that a diameter of a photosensitive drum is decreased, or a length of a photosensitive belt is shortened, and thus, respective apparatuses capable of performing respective image forming processes are arranged in the vicinity of these apparatuses, for instance, even when upset exposing timing, transferring timing cannot be displaced. As a result, as to shocks and vibrations caused by abutting operation of members, disturbances of images could not sufficiently suppressed in these conventional image forming apparatuses